The present invention relates to an image density control method for electrophotography, and more particularly to an image density control method of the type in which at least two latent-electrostatic-image test patterns which are significantly different from each other in the electric potential of the latent electrostatic images are formed on the surface of a photoconductor, and values relevant to the image density of each test pattern when developed are detected and, in accordance with the detected values, copy processing parameters which have an effect on the image density are controlled.
In conventional electrophotographic copying apparatus and electrostatic recording apparatus, latent electrostatic images formed on a latent-electrostatic-image-bearing member are developed by colored fine particles called toner, which toner is supplied from a development apparatus. The toner is electrically charged to a polarity opposite to the polarity of the latent electrostatic images, so that the toner particles are electrostatically attracted to the latent electrostatic images, whereby the above mentioned development is performed.
As a method of electrically charging toner to a polarity opposite to the polarity of latent electrostatic images, a method is known of mixing toner particles and carrier particles (which together constitute a developer), so that the toner particles and carrier particles are triboelectrically charged. Such a developer is generally called a two-component type developer. In a development method in which a two-component type developer is employed, the toner particles can be electrically charged easily and sufficiently for development. However, during the development process, only the toner particles are consumed, and variation in the relative concentrations of toner and carrier effect the image density. Therefore, a means for maintaining constant concentration of the toner in the developer is required, and, to do this, accurate measurement of the concentration of toner is indispensable.
A method of measuring the toner concentration of a developer is proposed in Japanese Patent Publication SHO 43-16199/1968. In that method, a standard latent electrostatic image pattern is formed on a photoconductor and the latent electrostatic image pattern is developed to a visible image pattern. The image density of the thus developed image pattern is photoelectrically measured, whereby the toner concentration of a developer is indirectly measured.
In addition to the above described indirect toner concentration measurement method, there are conventionally known a number of direct methods, for instance, a method of measuring the weight of a unit volume of the developer, and a method of measuring the permeability of the developer.
Furthermore, as a method for controlling image density relating to the toner concentration of a developer, the following methods have been proposed:
(1) A method of controlling a development bias voltage in accordance with the difference between the reflectance of a standard density plate and the reflectance of an original (Japanese Laid-Open Patent Application 53-103736/1978);
(2) A method of controlling development characteristics by detecting the image density during the process of copying a standard original (Japanese Laid-Open Patent Application SHO 54-141645/1979); and
(3) A method of controlling the charging of a photoconductor, development bias voltage and exposure by detecting the image density of an original, the potential of a latent electrostatic image, and the image density of developed image (U.S. Pat. No. 2,956,487).
Of the above mentioned recording image density control methods, in the case where an image is projected upon the surface of a photoconductor to form the corresponding latent electrostatic image thereon, and the latent electrostatic images are developed to visible image patterns, and the image density of each image pattern is detected by a photosensor, if toner particles are deposited on a light emission element and a light receiving element which constitute the photosensor, or if those elements have deteriorated, the input and output characteristics of the photosensor vary and correct control of the toner concentration cannot be attained.
In U.S. Pat. No. 4,082,445, there is disclosed a method of measuring image density, while correcting for changes in characteristics of a light receiving element and a photoconductor, if any, which are caused, for instance, by changes in power source voltage, toner deposition on the light receiving element, changes in ambient temperature, and deterioration of the light receiving element and photoconductor with time. In this method, the reflectance of a non-image area in a photoconductor, where no toner is deposited, is measured photoelectrically. Since the uncovered surface of a photoconductor should have its own constant reflectance, periodic reflectance measurement of the non-image area (or background) will indicate changes in characteristics of the photoconductor, if any. If the characteristics change, the electric current which flows through the light emission element is changed until the light receiving element outputs a normal voltage output. Thus, with the image density and output voltage characteristics of the light receiving element corrected, the image density of a standard image is measured, whereby the concentration of toner is controlled. In this method, however, it is necessary to cause relatively great electric current to flow through the light emission element. Therefore, a special circuit is necessary for that, which makes the apparatus using this method expensive. Furthermore, the life of the light emission element will be short due to the flowing of relatively great electric current therethrough.